The aeration problem

The presence of air in the fluid of a working hydraulic system can cause significant performance problems. Furthermore, the air may be in an entrained, or dissolved state which is difficult to detect. Only the most experienced hydraulic engineers can attribute lackluster system performance to the presence of air in the working fluid. Some of the ill effects of air in oil are illustrated in Fig. 1 [1]. Part of the difficulty in the diagnosis of air is that a reliable and practical air measurement device has never been made available outside of academia. A practical air measurement device would help hydraulic system developers and users assess and determine performance problems due to aeration. Hydraulic system modelers could also benefit greatly from such information by incorporating aeration data into their analytical models. Aeration is known to directly affect such fluid parameters as bulk modulus, density, and sonic velocity.

Figure 1. Some Results of Air in Hydraulic Oil.

How have we measured aeration in the past?

Methods such as sonic velocity and turbidity measurements have been investigated [2,3]. However, none of these methods could be developed for commercial use. In addition, the research conclusions state that these two methods are only applicable to measurement of entrained air. With no practical devices available, hydraulic engineers are forced to rely on their intuition and a qualitative visual inspection of the oil in the reservoir, which is not always easily accessible. Fig. 2 provides a typical qualitative correlation between visual appearance and the percent entrained air by volume for MIL-L-2104B hydraulic oil [1].

Figure 2. Qualitative Appearance of MIL-L-2104B with
Various Amounts of Entrained Air.

Is there a better way?

Fortunately, today a superior alternative exists that has the following advantages:

	light weight (10 lbs)
	compact dimensions width: 6.0 inches max height: 6.5 inches max
	retracted length: 23.0 inches
	extended length: 35.0 inches
	easy to read electronic display of results
	easy to operate
	capable of measuring entrained air and dissolved air
	wide operating range
	capable of sampling open reservoirs and pressurized lines
	extremely low maintenance and care
	rugged enough for day to day laboratory and field use

What is this device?

This device, shown in Fig. 3, is known as the Aeration Measuring Device (AMD). One of the most powerful aspects of the AMD is its ability to sample and evaluate oil from pressurized lines in an operating hydraulic system. The AMD utilizes a "mini slip-on gage probe" which works in conjunction with a "mini slip-on gage plug" (1/8" Male NPT connection) that you can install at any pressurized location (up to 2200 psig). Fig. 4 illustrates the AMD about to be attached to a gage plug which is installed in a typical hydraulic "tee" fitting. A screw on cap is provided with the gage plug to protect it when not in use. An optional high pressure probe and plug is available which extends the sampling pressure capablility to 5000 psi. Attaching the AMD to the plug only requires 12 pounds of force for every 1000 psi of system pressure. Fig. 5 demonstrates the simple attachment of the AMD to the plug.

How does it work?

The primary components of the AMD are a barrel and plunger, a manual crank, a digital scale, manually adjustable metering valve, pressure gauge for monitoring barrel pressure (140 psig max), and a slip on gage probe. The digital scale monitors the plunger position, and the manual crank is used to purge the translucent barrel of sample fluid so that the sampling procedure may be repeated. Fig. 6 is a mechanical drawing illustrating the primary components of the AMD. In summary, the sampling procedure requires the AMD to be purged of all air initially. With the valve closed, the AMD is attached to the hydraulic system as previously shown in Fig. 5. The digital display is set to zero and the metering valve is carefully opened. When the desired amount of sample is in the AMD, the valve is closed, the probe is removed from the gage plug, and the initial digital display reading is recorded. A vacuum is then created in the barrel by pulling back on the plunger using the crank handle. The vacuum separates the entrained and dissolved air from the oil. The operator then tilts the probe upward, opens the valve, and expels all air from the device. The final digital display reading is then recorded and compared to the initial reading for calculation of air present. The procedure is virtually the same for reservoir sampling, which is demonstrated in Fig. 7.

Figure 6. Mechancial Drawing Illustrating
Major Components of the AMD.

Figure 7. The AMD Engaged in Open Reservoir Sampling.

How do I know it really works?

The FES AMD has been evaluated using a special aeration test stand [3]. The test involved injecting a known amount of air into the system fluid and the fluid circulated until the conditions became uniform. The test was repeated for different aeration levels, with a de-aeration period between each test run. The following table shows the average results of four measurements taken at each aeration level to insure reliablility of the readings. As the fluid became more aerated, a reduction in sample size was necessary in order to be able to pull an adequate vacuum on the sample.

Average results of four measurements taken at each
aeration level to insure reliability of the readings

What about fluid compatibility?

The standard AMD is equipped with nitrile seals. Nitrile is compatible with most standard hydraulic fluids and water. At this time, it is not recommended that the AMD be used with phosphate ester based fluids or fuels. Development efforts are under way to establish the compatibility of the AMD wetted components with these types of fluids. Future versions of the AMD will be upgradeable to handle fuels, phosphate esters, and brake fluids.

How do I order an AMD package?

Contact FES Inc. or our local representative in your area. For information concerning local representation, please contact FES, Inc. at 5111 N. Perkins Road, Stillwater, OK 74075, telephone (405) 743-4337, fax (405) 743-2012, or E-mail HYPNEU@AOL.COM.

A note on usage and safety

Before attaching anything to a pressurized system, it is imperative to know exactly what the operating pressure will be at that location. When installing the gage plug, install an appropriate calibrated pressure gauge near the same location so that the working pressure is always known. NEVER exceed the rated working pressure of the brass gage plug (2200 psig) or the stainless steel gage plug (5000 psig). In addition, when collecting a fluid sample with the AMD from a pressurized source, carefully monitor the barrel pressure of the AMD using the pressure gauge provided. DO NOT EXCEED 140 psig barrel pressure. This can happen if the piston is allowed to reach end of stroke. The AMD is equipped with a safety rupture disk for protection should the pressure exceed 140 psig.

References

Elliot, L.R.,"Field Measurement of Hydraulic Fluid Aeration Level," Eighth Annual Fluid Power Research Conference, Oklahoma State University, Paper No. P74-31, October 8-9, 1974.

Elliot, L.R.,"Sonic Velocity Determination as a Measurement of the Entrained Air Present in Operating Fluid Power Systems," Eighth Annual Fluid Power Research Conference, Oklahoma State University, Paper No. P74-33, October 8-9, 1974.

January, D.B., and Tessmann, R.K., "The Use of a Turbidimeter to Measure Fluid Aeration Levels," Eight Annual Fluid Power Research Conference, Oklahoma State University, Paper No. P74-32, October 8-9, 1974.

Howsden, J.M., and Tessmann, R.K., "A Practical Device for Measuring Aeration Levels in Operating Hydraulic Systems," Fluid Power Research Conference, Oklahoma State University, Paper No. P75-15, October 7-8, 1975.

This page was last modified on 12/14/07
Copyright © 2007by FES, Inc., Stillwater, Oklahoma